1. Technical Field
The present invention generally relates to non-attached monitoring devices and antenna configurations for transmitting through an oriented attenuating body. More particularly, the present invention is related to a non-attached monitoring device that is free to move about within the cavity formed by the tire and the rim on which the tire is mounted. Specifically, the present invention relates to the configuration of the device antenna and its ability to transmit data through the attenuating body of the tire sidewall.
2. Background Information
Monitoring conditions of pneumatic tires while they are installed and in use on a particular vehicle is desired in the art. The users of this technology particularly desire to measure the internal temperature and internal pressure of a tire. These non-destructive measurements are preferably taken while the tire is in use without having to remove the tire from the vehicle or otherwise interrupt the use of the vehicle to take the measurements. It is particularly desirable to monitor the conditions and statistics of large off-the-road truck tires because the off-the-road tires are expensive and subject to harsher conditions than typical passenger car tires. Owners of highway trucks also desire to monitor their tires. Both types of tires must be regularly maintained to maximize vehicle usage and tire efficiency.
Numerous types of monitoring devices are known in the art. One type of known monitoring device uses a passive integrated circuit embedded within the body of the tire that is activated by a radio frequency transmission that energizes the circuit by inductive magnetic coupling. Other prior art devices used for monitoring tire conditions include self-powered circuits that are positioned external of the tire, such as at the valve stem. Other active, self-powered programmable electronic devices are disclosed in U.S. Pat. Nos. 5,500,065, 5,573,610, 5,562,787, and 5,573,611 which are assigned to the Assignee of the present application.
The prior art attachment problems exist because the forces on an electronic monitoring device while connected to a pneumatic tire are significant and numerous. The forces in the footprint area of the tire must be considered when mounting a monitoring device. Tires are subject to rotational forces when the vehicle is moving and also to various impact forces when the tire contacts surface irregularities. The tire will also deform and deflect during maneuvering because the loads to the wheel change. The attachment of the monitoring device to the tire must be strong enough and secure enough to maintain the position of the monitoring device with respect to the tire while experiencing all of these forces while also protecting the monitoring device from damage resulting from these forces. These concerns have lead to the use of non-attached monitoring devices such as those disclosed in U.S. Pat. No. 6,082,192 (commonly assigned) and U.S. Pat. No. 4,067,235.
One drawback with these devices is that their freedom of movement prevents the position of the antenna from being accurately predicted while the monitoring device is in use. The art recognizes that the position of the antenna with respect to the tire sidewall is an important factor when attempting to radiate a radio frequency signal through certain tire sidewall constructions. The rubber composite materials used in tire constructions may include a conductive material such as carbon black. These materials can attenuate the signal of some radio frequency transmissions through the tire sidewall or other tire structure. Some tire constructions such as those commonly used in certain highway truck tires and off-the-road tires can also include metal cords. For example, a plurality of metal cords oriented radially may be present in the sidewall area of these tires. Such metal structures will provide additional attenuation of a radio frequency signal passing through the tire sidewall. The orientation of the metal cords in the tire structure in relation to the orientation of the electromagnetic field associated with a radio signal passing through the tire sidewall will determine the degree of additional attenuation that the presence of the metal cords imparts upon the radio signal. Therefore, a tire structure of this type is termed an “oriented” attenuating body. The electromagnetic field orientation is a direct result of the type and orientation of antenna used in transmitting the radio signal. Certain antenna configurations provide more desirable transmissibility through the oriented attenuating body than other antenna configurations. One known antenna configuration is disclosed in U.S. Pat. No. 6,474,380 (commonly assigned) wherein a dipole antenna is fixed to the sidewall and disposed perpendicular to the metal body cords. This type of antenna configuration is not typically used with non-attached monitoring devices because the movement of the monitoring device does not allow the antenna to remain perpendicular to the metal body cords during all transmissions. Some prior art solutions have attempted to control the position of the non-attached monitoring device within the tire so that the antenna would be in a predictable position with respect to the tire sidewall. The art desires a monitoring device and antenna configuration that will provide signal transmissions through the tire sidewall regardless of the position of the monitoring device with respect to the tire sidewall.